Exhaust turbomachines, in particular exhaust turbochargers, are often provided to extract work from the exhaust stream of an internal combustion engine.
Exhaust turbochargers, which may be multi-stage, are typically used to boost air provided to the inlet tract of the engine. An exhaust turbocharger is often provided with a bypass whereby the turbine inlet and turbine outlet are directly connected; such a bypass is useful to prevent overspeeding of the turbocharger, and to reduce cold start time of an exhaust catalyser.
Another kind of exhaust turbomachine is an exhaust turbocompounder. Such a machine is typically used to extract work from the exhaust stream, and to provide mechanical, electrical or hydraulic power. A turbocompounder may for example be mechanically coupled to an engine crankshaft, or may drive an electrical generator.
Exhaust turbomachines have many benefits. They must however be used carefully, in particular to prevent an increase in back pressure on the engine, which could prevent effective scavenging of exhaust gases with a consequent reduction in engine performance.
GB-A-2423797 (Lotus) discloses a multi-cylinder internal combustion engine having two exhaust valves per cylinder, and the exhaust valves having respective exhaust manifolds connected one each to an exhaust turbocharger and to an exhaust turbocompounder. A variable valve train permits the opening duration of each exhaust valve to be varied, thereby changing the proportion of exhaust flow to each turbomachine, and promising better conversion of exhaust gas energy into useful work over the range of speed and load of the engine.
The present invention provides an improved arrangement of exhaust turbomachines capable of extracting work from an exhaust gas stream whilst maintaining efficient operation of the internal combustion engine.